ESCAMPIG XXI, Viana do Castelo, Portugal, July 10-14 2012
Topic number: 6
Numerical simulation of large area microwave plasma system for deposition of nanocrystalline diamond films A. Obrusn´ık1 , Z. Bonaventura(∗)1,2 , S. Potock´y3 , A. Kromka3 1
Department of Physical Electronics, Faculty of Science, Masaryk University, Kotl´arˇsk´a 2, CZ-61137 Brno, Czech Republic 2 R&D Center for Low-cost Plasma and Nanotechnology Surface Modifications, Masaryk University, Kotl´arˇsk´a 2, CZ-61137 Brno, Czech Republic 3 Institute of Physics of the ASCR, v.v.i., Cukrovarnicka 10, CZ-16253 Prague, Czech Republic (∗)
[email protected]
Simulation results for a linear antenna microwave device for deposition of nanocrystalline diamond thin films are presented. Electric field and the gas flow (for simplified gas composition) were simulated using COMSOL Multiphysics in order to determine the necessary plasma parameters of the discharge confined in the plasma source. Current work represents an important first step towards a fully self-consistent model of the linear antenna microwave plasma source.
A unique combination of physical and chemical properties such as high thermal conductivity, hardness, optical properties, chemical inertness or high affinity for covalent bonding with specific organic molecules makes nanocrystalline diamond (NCD) films a suitable material for various applications. [1, 2, 3]. The importance of large area deposition of NCD films with controllable morphology and intrinsic properties is growing and is still a nontrivial task due to the necessity of uniform growth conditions, i.e. plasma homogeneity over a large area. The linear antenna microwave (MW) technology [4] is considered one of the most promising solutions for large area deposition of NCD films. In order to improve the process of the plasma synthesis of nanocrystalline diamond thin films, it is necessary to study and understand plasma parameters and processes leading to the production of radicals in the plasma. Simulation of linear antenna microwave source (Figure 1) is a tremendous task due to sophisticated geometry and strong non-linear coupling between the heavy particles’ kinetics, plasma, electromagnetic waves, surface processes and energy exchange in such a complex, non-equilibrium medium. Self-consistent simulation provides a deep physical insight into discharge physics and therefore it is a very suitable tool for the optimization of the plasma source. This work focuses simulation of the electric field, the gas flow and mixing of the species (with simplified composition and chemical reaction channels) using COMSOL Multiphysics in (close-to-)real geometrical representation of the discharge chamber. The knowledge of the electric field distribution and the gas flow is essential for determining plasma parameters of the discharge that is confined in the reactor chamber. COMSOL’s RF module was used for simulating the electric field in the discharge chamber. The plasma behaves, with regard to the electric field, as a dielectric with imaginary permittivity and conductivity, both being a function of the plasma frequency and the electron-neutral collision frequency. The power absorbed by the plasma was the most important outcome of the electric field simulations as it was a necessary input for the gas flow and mixing models. For simulation of the gas flow and mixing of the species, the CFD Module was used. In the first approximation, the ionization and excitation processes due to the microwave electric field were neglected and only changes in macroscopic properties of the gases (viscosity, density, thermal conductivity, heat capacity) due to temperature increase and thermal dissociation were considered (Figure 2). The plasma was represented by a region, in which the microwave power dissipates, heating the gases. As previously mentioned, the amount and distribution of power dissipating in the plasma region were obtained from the electric field model. Consequently, using the Plasma module, the ionization and excitation of hydrogen were taken into account. The results obtained from these two simplified models are compared.
This study paves the way for fully self-consistent simulation of the operation of the plasma device with complex (H2 /CH4 /CO2 ) gas composition that will provide a deep physical insight into complex media which is used for NCD growth.
Fig. 1: Schematic drawing of the linear antenna pulsed MW plasma discharge chamber, lengthwise cross-section.
(a) Hydrogen velocity field
(b) Hydrogen temperature
Fig. 2: Results for hydrogen obtained from the simple thermodynamic model, input power 2x850W (66% duty), pressure 100 Pa, traverse cross-section
Acknowledgements We acknowledge financial support by P205/12/0908 (GACR). We would like to gratefully appreciate O. Rezek for technical assistance with preparation of samples, and Z. Polackova for wet chemical treatments.
References [1] D.M. Gruen, Annual Review of Materials Science 29 (1999) 211–259. [2] A. Hartl, E. Schmich, J.A. Garrido, J. Hernando, S.C.R. Catharino, S. Walter et al., Nature Materials 3 (2004) 736–742. [3] B. Rezek, D. Shin, H. Watanabe, C.E. Nebel, Sensors and Actuators B-Chemical 122 (2007) 596– 599. [4] A. Kromka, O. Babchenko, T. Izak, K. Hruska, B. Rezek, Vacuum 86 6 (2012) 776–779.
